1. Field of the Invention
The present invention relates to an optical waveguide grating formed in an optical waveguide such as an optical fiber, and a method of making the same.
2. Related Background Art
Various applications of optical waveguide gratings having a periodic refractive index changing area (grating area) along the optical axis of an optical waveguide to optical filters and the like in optical communication systems have conventionally been studied. Among others, those having a relatively long grating period of several hundreds of micrometers are known as long-period grating (see, for example, A. M. Vengsarkar, et al., J. Lightwave Tech., Vol. 14, No. 1, pp. 58-65, 1996), and their use in gain equalizers, band-stop filters, and the like is expected. Since characteristics of such a long-period optical waveguide grating have been known to vary with changes in temperature, temperature characteristics of optical waveguide gratings have been analyzed (see, for example, J. B. Judkins, et al., OFC ""96, PD-1, 1996).
That is, in an optical waveguide grating, SiO2 is used as a main ingredient of the optical waveguide, and GeO2 is generally added thereto in order to form a core region, which is a light-propagating area of the optical waveguide, and periodically generate a refractive index modulation within the core region, so as to form a grating area. Since the change in refractive index with respect to temperature, i.e., the temperature dependence of refractive index, is greater in GeO2 than in SiO2, the temperature dependence of refractive index in the core region and that in its surrounding cladding region differ from each other. As a result, in a long-period optical waveguide grating formed in such an optical waveguide, the temperature dependence of effective refractive index for core-propagating light and that for cladding-mode light differ from each other, whereby peak wavelengths would vary when temperature changes.
The temperature dependence of each of the respective refractive indices of SiO2 glass, GeO2 glass, and B2O3 glass has been known (O. V. Mazurin, et al., xe2x80x9cHandbook of Glass Data,xe2x80x9d Elsevier, 1985), and there has been known a technique based thereon in which, when an optical waveguide is a silica type optical fiber, its core region is co-doped with Ge element and B element, so as to lower the temperature dependence of characteristics of the optical waveguide grating (K. Shima, et al., OFC""97, FB2, 1997).
However, since the technique disclosed in the above-mentioned publication of Shima, et al., is based on the result of investigation of temperature dependence of refractive indices in three kinds of oxides (SiO2, GeO2, and B2O3) disclosed in the above-mentioned publication of O. V. Mazurin, et al., the optimal doping ratio can be adjusted only by combinations of these oxides, whereby the freedom in designing is limited. Also, since the infrared absorption edge of B2O3 is located on the shorter wavelength side from the respective infrared absorption edges of SiO2 and GeO2, the light absorption in the wavelength band of 1.55 xcexcm will increase by one digit or more if an optimal amount of B2O3 is added to the optical waveguide. For example, an optical fiber having a relative refractive index difference of about 1% in which the doping ratio of Ge element and B element have been optimized yields an absorption loss of 24 dB/km, whereby it would not be applicable to practical use. Hence, there has been no development of optical waveguide gratings having a low temperature dependence and low absorption loss in the wavelength band of 1.55 xcexcm, which is used for optical communications.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical waveguide grating having a low temperature dependence and yielding a low absorption loss in the wavelength band of 1.55 xcexcm, and a method of making the same.
Here, the temperature dependence of the peak wavelength xcexm of mode coupling in an optical waveguide grating having a grating period of xcex9 is represented by the following expression:                                           ⅆ                          λ              m                                            ⅆ            T                          =                  Λ          ⁡                      (                                                            ⅆ                                      n                    01                                                                    ⅆ                  T                                            -                                                ⅆ                                      n                    m                                                                    ⅆ                  T                                                      )                                              (        1        )            
Here, n01 is the effective refractive index of core-propagating light, nm is the effective refractive index of the m-th order cladding mode light, and T is the absolute temperature. Namely,       ⅆ          n      01            ⅆ    T  
is the temperature dependence of the core-propagating light, whereas       ⅆ          n      m            ⅆ    T  
is the temperature dependence of the m-th order cladding mode light. As can be seen from the above-mentioned expression, if the temperature dependence of effective refractive index for core-propagating light and that for cladding mode light can be caused to match each other, the temperature dependence of the peak wavelength xcexm in the optical waveguide grating can be lowered.
From such a viewpoint, the inventors have studied the temperature dependence of refractive indices of various glass-forming oxides. The glass-forming oxides studied here are constituted by four kinds, i.e., pure SiO2, SiO2 doped with 10% of GeO2, SiO2 doped with 10% of B2O3, and SiO2 doped with 10% of P2O5. For each of the glass-forming oxides, the refractive index difference xcex94n with reference to pure SiO2, and the temperature dependence of refractive index dn/dT were studied. The results are listed in the following Table 1.
As can be seen from Table 1, as with B2O3, the difference in temperature dependence dn/dT of P2O5 from pure SiO2 is negative. Further, since P2O5 has an infrared absorption edge located on the longer wavelength side from that of B2O3, the absorption loss in the wavelength band of 1.55 xcexcm would not deteriorate when it is added to silica glass. Consequently, the inventors have found it possible to lower the absorption loss in the wavelength band of 1.55 xcexcm while yielding a temperature dependence dn/dT substantially matching that of pure SiO2 if P2O5, in place of B2O3, is added to SiO2 together with GeO2.
The optical waveguide grating in accordance with the present invention is based on this finding, and is formed in an optical waveguide, mainly composed of SiO2, having a cladding region around a core region, having an area where a refractive index of the core region changes periodically along the optical axis direction; wherein the core region is co-doped with GeO2 and P2O5.
As a consequence of such a configuration, the temperature dependence dn/dT of the optical waveguide grating can substantially match that of pure SiO2 without adding B2O3, which may enhance the absorption loss, or while reducing the amount of addition thereof. Hence, while the absorption loss in the wavelength band of 1.55 xcexcm is suppressed, the temperature dependence of characteristics of the optical waveguide grating is lowered.
The molar doping amount of P2O5 in the core region is preferably {fraction (1/15)} to 1 times, more preferably 0.6 to 1 times that of GeO2. According to the inventors"" findings, the temperature dependence and absorption loss can favorably be lowered if the molar doping ratio is set as such. Also, since the doping amount can be selected from such a range, it increases the design flexibility, and it is easy to manufacture.
Alternatively, the core region may further be doped with B2O3, such that the sum of respective molar doping amounts of P2O5 and B2O3 is {fraction (1/15)} to 1 times the molar doping amount of GeO2, whereas the molar doping amount of B2O3 is smaller than that of P2O5. According to the inventors"" findings, if the molar doping amounts are set as such, then the increase in absorption loss in the wavelength band of 1.55 xcexcm, which is seen when B2O3 and GeO2 are doped without P2O5, can be suppressed, whereby the temperature dependence can favorably be lowered. Also, since the doping amount can be selected from such a range, it increases the design flexibility, and it is easy to manufacture.
Also, the cladding region may be doped with fluorine, and its refractive index may be adjusted so as to become lower than that of pure silica glass. As a consequence, the viscosity difference in core/cladding is lowered, so that the uniformity in processing improves at the time of processing a preform and drawing, whereby the ellipticity of the core is lowered, and an optical waveguide grating with a low polarization-dependent loss is obtained.
In this case, letting xcex94nxe2x88x92 be the ratio of decrease in refractive index of the cladding region with respect to pure silica glass, the molar doping amount of P2O5 in the core region is preferably adjusted within the range of (0.8-0.7xcex94nxe2x88x92)xc2x10.2 time that of GeO2. According to the inventors"" findings, when an F-doped cladding is employed, it is preferable for the doping amount to be set as such, in order to lower the temperature dependence of characteristics and the absorption loss, while matching the temperature dependence of the core and that on the cladding side. Also, since the doping amount can be selected from such a range, it increases the design flexibility and it is easy to manufacture.
On the other hand, the method of making an optical waveguide grating in accordance with the present invention is a method of making an optical waveguide grating of SiO2xe2x80x94GeO2xe2x80x94P2O5 based glass, in which, without loading the optical waveguide with hydrogen, or after loading the optical waveguide with hydrogen at a pressure of 20 atmospheres or less, said optical waveguide is irradiated with an ultraviolet ray having a wavelength ranging from 150 to 200 nm so as to form the grating.
In the conventional optical waveguide grating of SiO2xe2x80x94GeO2 based glass (further including B2O3), it has been necessary to load the optical waveguide with a large amount of hydrogen before making the grating; and, if hydrogen occluded in the grating is left as it is after the making, then thus occluded hydrogen may gradually be released to the outside of the optical waveguide, whereby characteristics of the grating may change. Therefore, it necessitates a hydrogen removing process, by which the grating wavelength may change. While the optical waveguide grating in accordance with the present invention comprises an SiO2xe2x80x94GeO2xe2x80x94P2O5 based glass as mentioned above, the inventors have found that a glass co-doped with GeO2 and P2O5 is also photosensitive to ultraviolet rays having a wavelength of at least 150 nm but not greater than 200 nm, and that the hydrogen loading process is totally or substantially unnecessary when the ultraviolet rays having a wavelength ranging from 150 to 200 nm are employed. Therefore, according to the manufacturing method of the present invention, the optical waveguide grating can favorably be made without being influenced by occluded hydrogen.
Alternatively, the method may be such that the optical waveguide is spot-heated periodically along the optical axis direction so as to form the grating. In the glass co-doped with GeO2 and P2O5, a stress is applied to the core/cladding interface due to the differences in viscosity and coefficient of thermal expansion in core/cladding. The inventors have found that a grating having desirable characteristics can favorably be made if this stress is periodically alleviated by thermal effects or if thermal diffusion of an additive is utilized. In this case, since the hydrogen loading process is totally unnecessary, there is no effect of such occluded hydrogen.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.